Vacuum method of manufacturing steel



W. ROHN VACUUM METHOD OF MANUFACTURING STEEL May 12, 1936..

Filed Nov. 20, 1933 2 Sheets-Sheet l mLHEL/W BY l ATTORNEYS May 12, 1936w. ROHN VACUUM METHOD OF MANUFACTURING STEEL Filed Nov. 20, 1953 2Sheets-Sheet 2 INVENTOR mHEL/W JQOH/V.

BY ATTORNEYS Patented May 12, 1936 PATENT OFFICE 2,040,568 VACUUM METHODOF MANUFACTURING STEEL Wilhelm ltolm, Hanau-on-the-Main, Germany,assignor to Heraeus-Vacuumschmelze A. G.,

Hanau-on-the-Main, Germany,

company a German Application November 20, 1933, Serial No. 698,923

j 11 Claims. (Cl. "is- 2) This invention relates to metallurgy and moreparticularly to methods of manufacturing low carbon iron alloys andsteels containing a proportion of one or more of the strong carbideformr 5 ing elements such as chromium, vanadium,

tantalum, manganese,'columbium, uranium, titanium and the like whoseoxides are diiflcultly reducible.

One of the objects of the present invention is to provide an improvedmethod for decarburizing such alloys and steels.

Another object of the present invention is to provide a method ofdecarburizing iron alloys and steels containing one or more of the aboveidentifled strong carbide-forming elements without the substantialoxidation of the said element or of the other constituents of the alloyor steel.

Another object of the present invention is to provide an improved methodfor obtaining low carbon iron and steel alloys containing'one or more'ofthe strong carbide-forming elements.

Other objects and advantages will be apparent as the invention isfurther disclosed.

In accordance with the objects of the present invention, 1 have foundthat molten iron or steel alloy baths containing one or more of thestrong carbide-forming elements may be successfully and efficientlydecarburized without substantial oxidation of the carbide-formingelement or of the remaining constituents of the iron or steel alloy, byadding to the bath a proportion of an oxygen-bearing material reducibleby carbon and thereafter maintaining the bath molten in an airatmosphere under a maintained reduced pressure, for a time intervalsuflicient to permit the interaction of the carbon content of the bathwith the said oxygen-bearing material and the resultant elimination ofthe carbon oxide gases from the metal bath into the maintained reducedatmosphere.

One of the features of the present invention is that the amount ofoxygen-bearing material added to the molten metal bath need be but inslight excess to that theoretically needed to combine with all of thecarbon in the bath.

Another feature of the present invention lies in the maintenance of areduced pressure of atmospheric gases over the molten bath during thedecarburizing reaction. The interaction of the carbon content of themolten metal bath and the carbon reducible oxygen-bearing material addedto the bath produces carbon oxides, which are evolved from the metalbath. The rate of evolution of these gases and hence the rate ofinteraction of the carbon and oxygen-bearing material is dependent uponthe pressure of the atmosphere on the surface of the bath and the rel- Iative proportion of carbon oxide gases in the said atmosphere.

By reducing the atmospheric pressure and by 5 maintaining the reducedpressure by means of a constantly operating evacuating means, theconcentration of carbon oxides in the atmosphere is maintained low inthe reduced pressure atmosphere. As a result, a favorable rate ofreaction 10 is obtained and the process of decarburization thereby isefliciently and economically facilitated.

In general, the precise low pressure employed may be widely variedwithout departing from the extent and scope of the present invention.When 5 the carbon content is relatively high in the molten metal bath,very slight reduction in pressures are needed to promote the rapidevolution of carbon oxide gases from the molten metal bath. However, asthe carbon content of the molten 2 metal bath decreases, lower pressuresare required to effect the removal of carbon oxide gases, until when thecarbon content of the metal bath approximates 1.0 percent pressures aslow as and lower than to millimeters of mercury and 25 when the carboncontent of the metal bath approximates 0.1% pressures as low as andlower than 20 to 25 millimeters of mercury may be employed. The last fewhundredths of a percent of carbon are difllcult to remove due it isbelieved 30 to the fact that the extent of dilution of carbon andoxygen-bearing material in the molten metal bath has become so great andthat solution pressure limits of carbon and oxygen in the molten metalbath-are approached. I have found, how- 35 ever, that while the rate ofreaction is materially slowed down with lowering carbon content in themetal bath, by the maintenance of reduced pressures below 20 to 25millimeters of mercury for prolonged intervals of time, the carbon con-40 tent of the molten bath may be economically reduced to amountsapproximating about .01 percent.

Following decarburization of the molten metal bath, as above described,atmospheric pressure 45 may be restored and the molten metal may be castin any desired manner. Alternatively, the decarburized metal bath may bedegasified by heating inhigh vacuum, if desired. Vacuum' casting of themolten metal bath may be also em- 50 ployed, if desired.

There are many types of furnaces, evacuation pumps, and means forheating the molten metal bath that may be adapted to the practice of themethod of the present invention, and such furnaces, pumps. and means toheat the metal bath do not form a part of the present invention. Iprefer, however,

to utilize in the practice of the present invention aninduction furnaceof the general type indicated in the accompanying drawings wherein:

Fig. 1 is a side elevational view of a ring-type inductively heatedvacuum furnace; Fig. 2 is a top plan view with the cover removed showingthe relative positioning of the parts thereof; Fig. 3 is a second viewof the same with means to cast the molten metal in vacuum; and Fig. 4 isa schematic view illustrating the internal structure of the furnace.

As a specific embodiment of the practice of the present invention, themethod which has been devised to effect the decarburization of an alloyor steel containing chromium will be disclosed. The alloy or steel isplaced in the ring-shaped crucible 2 (Fig. 1) of the furnace i (Fig. l)and a proportion of metal oxide readily reducible by carbon is addedthereto. The alloy or steel may be either molten or in solid form, whenplaced in crucible 2.

Cover 3 is then placed over the furnace and sealed thereto at edges 4 inany convenient manner, and air is exhausted from the furnace throughpipe 5 by means of vacuum pump 6.

The metal in crucible 2 is then heated preferably by induced electricalcurrents from induction means 1 (and cooperating electrical apparatusand circuits not shown), which means includes primary windings 1 andyokes 9 to the temperature at which interaction between the carbon ofthe metal oxide is obtained.

This temperature of reaction or the required temperature to bring aboutthe evolution of carbon monoxide from the bath will vary somewhatdepending upon bath composition, the specific metal oxide added, theproportion of carbon present in the bath, the specific reduced pressureemployed, and the temperature of the bath.

with most types of chromiumfollowing materials; iron oxide, nickeloxide, chrome oxide, or various ores, containing one or more of theseoxides. It is preferable when ore materials are used to employ oreconcentrates which are substantially free from associated silica organgue impurities. However, if desired, I may admix with these materialone or more of the heretofore well known and so-called slagging"materials, such as fluorspar for example, which may facilitate thecontacting of the oxidizing material with the molten metal bath.

In general and with most chromium-containing alloys and steels, I preferto gradually reduce the pressure from atmospheric pressures to about 200millimeters and to maintain this pressure until the carbon content ofthe alloy has been reduced to about one percent. Higher or lower employa reduced pressure which facilitates the rate of interaction of thecarbon of the bath with the added reducible oxidizing material, butwhich controls the rate of evolution of the gas from the metal bath.During the initial stages of decarburization, the precise pressureemployed must be adjusted with respect to the evolution of gas toprevent .undue frothing or boiling of the metal bath. With oxidizingmaterials that are more bath and the added metal dimcultly reduced orwhen the reducing reaction has proceeded nearly to completion it isnecessary to use lower pressures than when the oxidizing material ismore readily reduced or during the initial stages of decarburization.After the carbon content of the alloy has been reduced to below about0.1 percent, the pressure may be reduced to 20 to 25 millimeters orlower.

from the molten metal bath. Such circulation of atmospheric gasesthrough the furnace may be obtained in any convenient manner as byoperation of vacuum means 6 while regulating valved opening in in cover3 permitting the entrance of atmospheric gases to the cover 3.

As an example of the invention, a 600 pound charge of high-carbonferrochrome (carbon approximately 5%, chromium approximately 60%) may bedecarburlzed in accordance with the present invention by heating totemperatures approximating 1750-l850 C., adding thereto chromium oxidein amounts in theoretically computed to combine with the carbon,reducing the pressure gradually to about 50 millimeters until the carboncontent of the alloy approximates 0.1%, then content approximates .01percent. The rate of decarburizatlon may vary within wide limitsdepending upon the conditions existing. In one experiment, the carboncontent was reduced from about .28 percent to.06 percent in about 2hours running time. After one and one half hours running time, thecarbon content was reduced from .06 to about .01 percent. If theresultant dilution of the chromium is not deleterious, otheroxygen-carrying materials such as iron oxides, mill scale, nickel oxideand the like may be employed instead of chromium oxide.

As a result of the practice of the present invention, the productobtained is a low carbon ferro-chrome substantially free from metaloxides practice of the present by a simple, economical refining ordecarburizing v process applied to high carbon farm-chromium. Heretoforein the art, this has been difficult to obtain. This low-carbonferro-chromium may be added to a low carbon steel or to a low carbonnickel-iron alloy in the forming of low carbon chrome steel or lowcarbon iron-nickel-chromium alloys. (Chromium steel and nickel chromiumsteel may be treated by the same method.)

The product from the decarburizing process, if it be a low-carbonferro-chrome, may be cast at atmospheric pressure in the customarymanner. or where the product is a low-carbon steel which is desiredsubstantially gas-free, the heating may be continued for a time intervalunder high vacuo and then cast either at atmospheric pressures or invacuo. To cast the alloy in vacuo, the cover 3 may be provided with twooppositely positioned water-cooled ingot molds I5 into which the moltenmetal may be poured directly from the crucible 2 by tilting the furnacein the proper direction.

As a further step, if desired, after degasification in high vacuum, themolten metal may be allowed to solidify in situ in the crucible, therebyexpelling the gases held in solution at the temperature of heating.Thereafter on reheating to fusion in vacuo the expelled gases areeifectivelv elimihated and the thus vacuum cast metal is of lower gascontent than that obtained without solidification and remelting. Ifdesired, the solidification and remelting may be repeated.

In general, however, and for most purposes chromium-containing iron andsteel alloys decarburized in accordance with the present invention donot require a pressure lower than about 20 to g 25 millimeters to eifectdecarburization down to a few hundredths of a percent carbon andsubstantial degasiflcation of the decarburized metal is obtained atthese pressures. This decarburized alloy may then be cast at atmosphericpressures in externally positioned molds with good results. It is onlywhere special uses or special properties aredesired in the alloy thathigh vacuum treatment and casting and solidification in vacuo isrequired. The ordinary residual gas content of a decarburized alloy maybe generally taken care of by slight additions of heretofore employeddegasifying elements such as titanium, zirconium, aluminum and the like.

It is therefore apparent from the above description and drawings of thepresent invention that many modifications and departures may be madetherein without departing from the nature and scope thereof and thatwhereas as a specific embodiment :I have described the same as it hasbeen applied to ferro-chromium I am not to be limited thereto as theprocess is applicable with only slight modification to all ferro-alloysof the strong carbide forming elements defined and to steel alloyscontaining said elements. All such modifications and departures arecontemplated as may fall within the scope of the following claims.

What I claim is:

1. The method of decarburizing iron and steel alloys containing at leastone of the strong carbide-forming elements, chromium, titanium, uraniumand the like which comprises heating the said steel to temperaturesabove the melting point, adding thereto a carbon-reducible oxidizingmaterial, enclosing the molten steel bath from the atmosphere,circulating atmospheric gases over the surface offlthe bath andcontinuing the heating of the bath under gradually reducing pressures ofsaid atmospheric gases until the carbon content of the bath has beenlowered to the desired minimum.

2. The method of claim 1, in which the molten bath is heated by inducedelectrical currents adapted to vigorously stir the bath while heatingthe same to maintain the said carbon-reducible material uniformlydispersed throughout the said bath.

3. The method of claim 1, in which the pressure of said atmosphericgases is gradually reduced from atmospheric pressures to approximately20 to 25 millimeters of mercury as the carbon content of said bathreducesto amounts approximating 0.1 per cent.

4. The method of claim 1, in which the bath is maintained at atemperature favorable to the reduction of said carbon-reducibleoxidizing ma.- terial by the carbon of the molten metal bath.

5. The method of claim 1, in which the amount of carbon-reduciblematerial added is proportioned with respect to the amount of carbon inthe molten metal bath to be theoretically in but slight excess thereto.-

6. The method of claim 1, in which the oxidizing material addedcomprises carbon-reducible metal oxides.

7. The method of claim 1, in which a proportion of slagging materials isadded to the said oxidizing material to facilitate the contactin of thesaid oxidizing material with the molten metal bath.

8. The method of claim 1, in which the pressure of. said atmosphericgases is gradually reduced from atmospheric pressures to pressures atleast approximating 15 to 35 millimeters of mercury.

9. The method of claim 1, in which the pressure of said atmosphericgases is reduced from atmospheric pressures to about 50 millimeters ofmercury in a gradual manner and maintained at this pressure until thecarbon content of the bath has been lowered to about 0.10 per cent andthen reduced further to about to 20 millimeters of mercury until thecarbon content has been lowered the desired amount below 0.10 per cent.

10. The method of decarburizing high carbon ferro-chromium alloys whichcomprises heating the said alloy to temperatures approximating 1750 to1850 0., adding thereto chromium oxidein amounts in slight excess tothat theoretically computed'to combine with the carbon, enclosing themolten bath from the atmosphere, circulating atmospheric gas over thesurface of the bath, and continuing the heating of the bath undergradually reducing pressures of said atmospheric gases until the carboncontent of the bath has been lowered to the desired amount, the saidpressures being reduced gradually from atmospheric pressures to about 50millimeters of mercury until, the carbon content approximates 0.10 percent and then' being reduced gradually to between 10 and 20 millimetersand held there until the carbon content has been reduced to the desiredpercentage below 0.10 per cent.

11. The method of claim 10, in which the heating of the bath to thedesired temperature is obtained by means of induced electrical currentsadapted to maintain the bath in vigorous circulation at least sufllcientto obtain uniform dispersion of the said chromium oxide throughout thebath.

WILHELM ROHN.

